This study examined the correlation of echocardiography (ECHO) and Cardiac Magnetic Resonance (CMR) in the assessment of aortic valve regurgitation (AR) in children and young adults with congenital heart disease. We hypothesized that qualitative ECHO assessment correlates insufficiently with quantitative CMR data and compared subjective ECHO evaluations with objective measurement of regurgitant fractions (RF) by CMR. Patients who had both ECHO and CMR assessments of AR within 60 days of each other were included. The qualitative ECHO assessment (mild, moderate, severe) of AR and left ventricular dimension at end diastole were recorded. RF was quantified by CMR using phase-contrast velocity mapping. Repeat ECHO review and grading of AR was performed by a blinded single reader in a randomly chosen subgroup of patients. In 43 patients studied, statistical significance was observed in the CMR-RF between mild and moderate, and between mild and severe ECHO grades. There was significant overlap of objective RF between subjective grades. Mild ECHO AR corresponded to an RF (%) of 0-29, moderate 1-40, and severe 5-58. Overlap was more significant at moderate and severe grades. Results were similar in the group in whom a single reader interpreted the ECHO assessment. In conclusion, results derived from a real-life multiple-reader ECHO laboratory showed inconsistencies in ECHO grading of AR, with a wide range of objectively measured RF within a given ECHO grade. ECHO is less reliable in identifying more severe AR, often overestimating severity. Quantitative CMR is a potentially useful supplement to ECHO for management decisions and assessments of medical and surgical therapies in children and young adults with AR.